This invention relates to methods of semiconductor fabrication and devices formed thereby.
2. Description of Related Art
As integrated circuit technology advances, the gate widths of MOSFETS become increasingly smaller. In addition, the thicknesses of the gate dielectrics, typically, gate oxides, become thinner and thinner. Very thin gate oxides are necessary for small CMOS transistors.
Often, ion implantation with boron BF.sub.2 is used to dope the polysilicon gates of surface channel PMOSFETS. However, as gate oxides become increasingly thinner, boron penetration through the gate oxide by diffusion in subsequent thermal cycles becomes a problem. Typically, the boron penetrates the gate oxide, doping the channel and adversely affecting the device threshold voltage, short channel effects, and circuit performance.
A variety of process steps have been tried to reduce boron penetration through the gate oxide. Some of these approaches have utilized nitrogen in an effort to reduce boron penetration through the gate oxide.
For example, after blanket layers of gate oxide and polysilicon have been deposited (but before the gate has been defined) nitrogen has been implanted into the polysilicon layer. It is hoped the nitrogen blocks the subsequently implanted boron. This approach is detailed in: T. Kuroi et al. 1994 Symp. on VLSI Tech., p. 107.
Another approach involves growing the gate oxide in an ambient of N.sub.2 O. This approach is detailed in Lo et al., IEEE Electron Device Letters, Volume 13, p. 111, February 1992.
A third approach involves growing a conventional gate oxide by conventional processes and then annealing the gate oxide in a nitrogen ambient. This approach is detailed in: K. Kirsch et al., IEEE IEDM, p. 325, 1994.
Nevertheless, those concerned with the development of integrated circuits processes still continually search for improved methods of reducing boron penetration.
As device dimensions scale down rapidly with the advance of VLSI technologies, the electric field in the thin gate oxides continues to increase. Part of the consequence of such increased electric field is the increased trap generation at the oxide interface or within the thin oxides. The trap generation and the capture of channel electrons by the traps in turn leads to increased low frequency (l/f) noise and transconductance (g.sub.m) degradation. For ultra-thin oxides of less than 50 .ANG., the tunneling current also becomes significant and gives rise to accelerated degradation of the device characteristics.